This invention relates to the detection of digital signals which have a rate of occurrence greater than a given value. The invention has particular application to the detection of error rates above a pre-selected or pre-determined threshold, e.g. in a serial stream of bits, the occurrence of x error bits in a window defined as a group of n bits.
The occurrence of error bits can be detected in a serial stream of bits by counting the number of such bits which occur within each successive group of n bits. Such a technique does not identify every occurrence of x bits within a group of n successive bits since the n bit windows are static so that if, for example, (x-1) bits occur at the end of one window and the next error bit occurs at the beginning of the next window, such an occurrence may not be identified. The present invention is concerned with arrangement which does not have such a limitation.